High-Efficiency GaAs-Based Solar Cells
The III-V compound solar cells represented by GaAs solar cells have contributed as space and concentrator solar cells and are important as sub-cells for multi-junction solar cells. This chapter reviews progress in III-V compound single-junction solar cells such as GaAs, InP, AlGaAs and InGaP cells. Especially, GaAs solar cells have shown 29.1% under 1-sun, highest ever reported for single-junction solar cells. In addition, analytical results for non-radiative recombination and resistance losses in III-V compound solar cells are shown by considering fundamentals for major losses in III-V compound materials and solar cells. Because the limiting efficiency of single-junction solar cells is 30-32%, multi-junction junction solar cells have been developed and InGaP/GaAs based 3-junction solar cells are widely used in space. Recently, highest efficiencies of 39.1% under 1-sun and 47.2% under concentration have been demonstrated with 6-junction solar cells. This chapter also reviews progress in III-V compound multi-junction solar cells and key issues for realizing high-efficiency multi-junction cells.
- solar cells
- III-V compounds
- high efficiency
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The III-V compound solar cells represented by GaAs solar cells have advantages such as high-efficiency potential, possibility of thin-films, good temperature coefficient, radiation-resistance and potential of multi-junction application compared crystalline Si solar cells. The III-V compound solar cells have contributed as space and concentrator solar cells and are important as sub-cells for multi-junction solar cells. As a result of research and development, high-efficiencies [1, 2] have been demonstrated with III-V compound single-junction solar cells: 29.1% for GaAs, 24.2% for InP, 16.6% for AlGaAs, and 22% for InGaP solar cells. Figure 1 shows historical record-efficiency of GaAs, InP, AlGaAs and InGaP single-junction solar cells along with their extrapolations .
The data can be fitted with the Goetzberger function :
where η(t) is the time-dependent efficiency, ηlimit is the practical limiting efficiency, t0 is the year for which η(t) is zero, t is the calendar year, and c is a characteristic development time. Fitting of the curve was done with three parameters which are given in Table 1. The extrapolations show that the progress of efficiencies is converging or will converge soon, which is mainly bounded by the Shockley-Queisser limit .
Fitting parameters for various solar cells.
Figure 2 shows calculated and obtained efficiencies of single-junction single-crystalline and polycrystalline solar cells . Because the limiting efficiency of single-junction solar cells is 30-32% as shown in Figure 2, multi-junction solar cells have been developed and InGaP/GaAs based 3-junction solar cells are widely used in space. Recently, highest efficiencies of 39.2% under 1-sun and 47.1% under concentration have been demonstrated with 6-junction solar cells .
This Chapter reviews progress in III-V compound single-junction solar cells such as GaAs, InP, AlGaAs and InGaP cells. In addition, analytical results for non-radiative recombination and resistance losses in III-V compound solar cells by considering fundamentals for major losses in III-V compound materials and solar cells. This chapter also reviews progress in III-V compound multi-junction solar cells and key issues for realizing high-efficiency multi-junction cells.
Analysis of non-radiative recombination and resistance losses of single-junction solar cells
By using our analytical model [8, 9], potential efficiencies of various solar cells are discussed. This model considers the efficiency loss such as non-radiative recombination and resistance losses, which are reasonable assumption because conventional solar cells often have a minimal optical loss. The non-radiative recombination loss is characterized by external radiative efficiency (ERE), which is the ratio of radiatively recombined carriers against all recombined carriers. In other words, we have ERE = 1 at Shockley-Queisser limit . EREs of state-of-the-art solar cells can be found in some publications such as references [2, 10, 11, 12, 13]. In this chapter, the EREs of various solar cells are estimated by the following relation :
where Voc the measured open-circuit voltage, k the Boltzmann constant, T the temperature, and q the elementary charge. Voc:rad the radiative open-circuit voltage and is expressed by the following Eq. 
where [Jph]Voc,rad is the photocurrent at open-circuit in the case when there is only radiative recombination and Jo,rad the saturation current density in the case of radiative recombination.
0.28 V for Eg/q. Voc;rad value reported in [15, 16, 17] were used in our analysis. Where Eg is the bandgap energy. The second term on the right-hand side of Eq. (2) is denoted as Voc;nrad, the voltage-loss due to non-radiative recombination and is expressed by the following Eq. .
where Jrad(V0c) is the radiative recombination current density and Jrec(Voc) is the non-radiative recombination current density.
Figure 3 shows open-circuit voltage drop compared to Band gap energy (Eg/q – Voc) and non-radiative voltage loss (Voc,nrad) in GaAs, InP, AlGaAs and InGaP solar cells [2, 8, 9, 10, 11, 12, 13, 17] as a function of ERE. High ERE values of 22.5% and 8.7% have been observed for GaAs and InGaP, respectively compared to InP (0.1%) and AlGaAs (0.01%).
The resistance loss of a solar cell is estimated solely from the measured fill factor. The ideal fill factor FF0, defined as the fill factor without any resistance loss, is estimated by .
The measured fill factors can then be related to the series resistance and shunt resistance by the following Eq. :
where rs is the series resistance, and rsh is the shunt resistance normalized to RCH. The characteristic resistance RCH is defined by 
r is the total normalized resistance defined by r = rs rsh −1.
Figure 4 shows correlation between fill factor and resistance loss [2, 8, 9, 10, 11, 12, 13, 17] in GaAs, InP, AlGaAs and InGaP solar cells. Lower resistance losses of 0.01-0.03 have been realized for GaAs, InP and InGaP solar cells compared to 0.05 for AlGaAs.
Historical progress and key issues for high-efficiency III-V compound single-junction solar cells
Table 2 shows major losses, their origins and key technologies for improving efficiency . There are several loss mechanisms to be solved for realizing high-efficiency III-V compound single-junction solar cells. (1) bulk recombination loss, (2) surface recombination loss, (3) interface recombination loss, (4) voltage loss, (5) fill factor loss, (6) optical loss, (7) insufficient –energy photon loss. Key technologies for reducing the above losses are high quality epitaxial growth, reduction in density of defects, optimization of carrier concentration in base and emitter layers, double-hetero (DH) structure junction, lattice-matching of active layers and substrate, surface and interface passivation, reduction in series resistance and leakage current, anti-reflection coating, photon recycling and so forth.
|Bulk recombination loss||Non radiative recombination centers (impurities, dislocations, grain boundary, other defects)||High quality epitaxial growthReduction in density of defects|
|Surface recombination loss||Surface sates||Surface passivationHeteroface layerDouble hetero structure|
|Interface Recombination loss||Interface statesLattice mismatching defects||Lattice matchingInverted epitaxial growthWindow layerBack surface field layerDouble hetero structureGraded Band-gap layer|
|Voltage loss||Non radiative recombinationShunt resistance||Reduction in density of defectsThin layer|
|Fill factor loss||Series resistanceShunt resistance||Reduction in contact resistanceReduction in leakage current,Surface, interface passivation|
|Optical loss||Reflection lossInsufficient absorption||Anti-reflection coating, textureBack reflector, photon recycling|
|Insufficient-energy photon loss||Spectral mismatching||Multi-junction (Tandem)Down conversionUp conversion|
Major losses, their origins of III-V compound cells and key technologies for improving efficiency.
Solar cell efficiency is dependent upon minority-carrier diffusion length (or minority-carrier lifetime) in the solar cell materials as shown in Figure 5.
Radiative recombination lifetime τrad is expressed by
where N is the carrier concentration and B is the radiative recombination probability. The B value for GaAs reported by Ahrenkiel et al.  is B = 2 X 10 −10 cm 3 /s. Effective lifetime τeff is expressed by
where τnonrad is non-radiative recombination lifetime and given by
where σ is capture cross section of minority-carriers by non-radiative recombination centers, v is minority-carrier thermal velocity, and Nr is density of non-radiative recombination center.
Therefore, improvement in crystalline quality and reduction in densities of defects such as dislocations, grain boundaries and impurities that act as non-radiative recombination centers are very important for realizing high-efficiency solar cells.
In this chapter, analytical results for historical progress in efficiency of GaAs single-junction solar cells are shown. Figures 6 and 7 show analytical results for progress in ERE and resistance loss of GaAs single-junction solar cells.
The first GaAs solar cells reported by Jenny et al.  were fabricated by Cd diffusion into an n-type GaAs single crystal wafer. Efficiencies of 3.2-5.3% were quite low due to deep junction. Because GaAs has large surface recombination velocity S of around 1 × 10 7 cm/s [6, 21], formation of shallow homo-junction with junction depth of less than 50 nm is necessary to obtain high-efficiency. Therefore, hetero-face structure AlGaAs-GaAs solar cells have been proposed by Woodall and Hovel  and more than 20% efficiency has been realized  in 1972 as shown in Figure 1 as a result of ERE improvement from 10 −8 % to 0.05% as shown in Figure 6. Double-hetero (DH) structure AlGaAs-GaAs-AlGaAs solar cell with an efficiency of 23% has been realized by Fan’s group in 1985  as a result of ERE improvement from 0.05% to 1.4% as shown in Figure 6. Now, DH structure solar cells are widely used for high-efficiency III-V compound solar cells including GaAs solar cells.
Figure 8 shows device structures of GaAs solar cells developed historically. As mentioned above and shown in Figure 8, device structures of GaAs cells were improved from homo-junction, to heteroface structure, finally to DH structure. Now, InGaP layer is mainly used as front window and rear back surface field (BSF) layers instead of AlGaAs layer. The reasons are explained in the part of multi-junction solar cells.
Figure 9 shows the chronological improvements in the efficiencies of GaAs solar cells fabricated by LPE (Liquid Phase Epitaxy), MOCVD (Metal-Organic Chemical Vapor Deposition) and MBE (Molecular Beam Epitaxy). LPE was used to fabricate AlGaAs-GaAs heteroface solar cells in 1972 because it produces high-quality epitaxial film and has a simple growth system. Homo-junction structure and heteroface structure GaAs solar cells shown in Figure 8 were fabricated by LPE. However, it is not as useful for devices that involve multilayers because of the difficulty of controlling layer thickness, doping, composition and speed of throughput. Since 1977, MOCVD has been used to fabricate large-area GaAs solar cells by using DH structure shown in Figure 8 because it is capable of large-scale, large-area production and has good reproducibility and controllability.
Regarding the differences of surface recombination velocities in semiconductor materials, differences of point defect behavior are thought to be one of the mechanisms. For example, because nearest-neighbor hopping migration energies (0.3 eV and 1.2 eV) of VIn and VP in InP  are lower than those (1.75 eV) of VGa and VAs in GaAs, better surface state may be formed on InP surface compared to GaAs surface.
In addition to improvement in surface recombination loss, as a result of technological development, resistance loss has been improved as shown in Figure 7. In parallel, bulk recombination loss and interface recombination loss have been improved as shown in Figure 6. Recently, efficiency of GaAs solar cells reached to 29.1%  by realizing ERE of 22.5% as a result of effective photon recycling .
Lattice mismatching also degrades solar cell properties by increase in interface recombination velocity as a result of misfit dislocations and threading dislocations generation. By using interface recombination velocity SI as a function of lattice mismatch (Δa/a0) for InGaP/GaAs heteroepitaxial interface , lattice mismatch (Δa/a0) dependence of interface recombination velocity (SI) is semi-empirically expressed by .
As one of example for effects of interface recombination loss upon solar cell properties, analytical results for correlation between ERE and interface recombination velocity in InGaP single-junction solar cells are shown in Figure 10.
Historical progress and key issues for high-efficiency III-V compound multi-junction solar cells
While single-junction cells may be capable of attaining AM1.5 efficiencies of up to 30-32% as shown in Figure 2, the multi-junction (MJ) structures [26, 27] were recognized early on as being capable of realizing efficiencies of up to 46% as shown below. Figure 11 shows the principle of wide photo response using MJ solar cells for the case of a triple-junction cell. Solar cells with different bandgaps are stacked one on top of the other so that the cell facing the Sun has the largest bandgap (in this example, this is the InGaP top cell). This top cell absorbs all the photons at and above its bandgap energy and transmits the less energetic photons to the cells below. The next cell in the stack (here the GaAs middle cell) absorbs all the transmitted photons with energies equal to or greater than its bandgap energy, and transmits the rest downward in the stack (in this example, to the Ge bottom cell). As shown in Figure 12, the spectral response for an InGaP/GaAs/Ge monolithic, two-terminal triple-junction cell shows the wideband photo response of multijunction cells. In principle, any number of cells can be used in tandem.
As a result of research and development, high-efficiencies have been demonstrated with III-V multi-junction solar cells: 37.9% under 1-sun and 44.4% under concentration for 3-junction cells  and 39.2% under 1-sun, 47.1% under concentration for 6-junction solar cells . Figure 13 shows historical record-efficiency of III-V multi-junction (MJ) and concentrator MJ solar cells in comparison with 1-sun efficiencies of GaAs and crystalline Si solar cells, along with their extrapolations .
Table 3 shows key issues for realizing super high-efficiency MJ solar cells. The key issues for realizing super-high-efficiency MJ solar cells are (1) sub cell material selection, (2) tunnel junction for sub cell interconnection, (3) lattice-matching, (4) carrier confinement, (5) photon confinement, (6) anti-reflection in wide wavelength region and so forth. For concentrator applications by using MJ cells, the cell front contact grid structure should be designed in order to reduce the energy loss due to series resistance (resistances of front grid electrode including contact resistance, rear electrode, lateral resistance between grid electrodes) by considering shadowing loss attributed to grid electrode, and tunnel junction with high tunnel peak current density is necessary. Because cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells in order to reduce losses of electrical connection and optical absorption, effectiveness of double hetero structure tunnel diode is also presented in this chapter.
|Top cell materials||AlGaAs||InGaP||AlInGaP|
|Middle cell materials||None||GaAs, InGaAs||GaAs, quantum well, quantum dots, InGaAs, InGaAsN etc.|
|Bottom cell materials||GaAs||Ge, InGaAs||Si, Ge, InGaAs|
|Substrate||GaAs||Ge||Si, Ge, GaAs, metal|
|Tunnel junction (TJ)||Double hetero structure-GaAs TJ||Double hetero structure-InGaP TJ||Double hetero structure-InGaP or GaAs TJ|
|Lattice matching||GaAs middle cell||InGaAs middle cell||(In)GaAs middle cell|
|Carrier confinement||InGaP-BSF||AlInP-BSF||Wide-gap-BSFQuantum dots|
|Photon confinement||None||None||Back reflector, Bragg reflector, quantum dots, photonic crystals, etc.|
|Others||None||Inverted epitaxial growth||Inverted epitaxial growth, epitaxial lift off|
Key issues for realizing super-high-efficiency III-V compound multi-junction solar cells.
Selection of sub-cell layers by considering optimal bandgap and lattice matching of materials is one of key issues for realizing super high-efficiency MJ cells. Table 4 shows one example for selection of top cell material and comparison of InGaP and AlGaAs as a top cell material. InGaP that has better interface recombination velocity, less oxygen-related defect problems and better window material AlInP compared to those of AlGaAs has been proposed as a top cell material by NREL group . As described above, InGaP materials are now widely used as front widow and back surface filed layers of solar cells instead of AlGaAs.
|Interface recombination velocity||10 4 –l0 5 cm/s|
|Window Layer (Eg)||AlInP (2.5 eV)||AlGaAs (2.1 eV)|
|Other problems||High doping in p-AlInP||Lower efficiency (2.6% lower)|
Comparison of InGaP and AlGaAs as a top cell material.
Figure 14 shows the connection options for two-junction cells: the two cells can be connected to form either two-terminal, three-terminal or four-terminal devices. In a monolithic, two-terminal device, the cells are connected in series with an optically transparent tunnel junction intercell electrical connection. In a two-terminal structure, only one external circuit load is needed, but the photocurrents in the two cells must be equal for optimal operation. Key issues for maximum-efficiency monolithic cascade cells (two-terminal multijunction cells series connected with tunnel junction XE “tunnel junctions”) are the formation of tunnel junctions of high performance and stability for cell interconnection, and the growth of optimum bandgap top- and bottom-cell structures on lattice-mismatched substrates, without permitting propagation of deleterious misfit and thermal stress-induced dislocations.
As shown in Table 3, cell interconnection of sub-cells is one of the most important key issues for realizing high-efficiency MJ solar cells. DH structure has been found to effectively prevent from impurity diffusion from tunnel junction and high tunnel peak current density has been obtained by the authors [30, 31]. Figure 15 shows annealing temperature (equivalent to growth temperature of top cell layers) dependence of tunnel peak current densities for double hetero structure tunnel diodes. X is the Al mole fraction in AlxGa1-xAs barrier layers [30, 31]. It has also been found that the impurity diffusion from the tunnel junction is effectively suppressed by the wider bandgap material tunnel junction with wider bandgap material-double hetero (DH) structure . These results are thought to be due to the lower diffusion coefficient for impurities in the wider Band gap materials such as the AlInP barrier layer and InGaP tunnel junction layer .
As a result of developing high performance tunnel junction with high tunnel peak current density, high efficiency MJ solar cells have been developed [30, 33, 34]. Figure 16 shows a structure and light-illuminated (AM1.5G 1-sun) I-V characteristics of InGaP/GaAs/InGaAs 3-junctuon solar cell. 37.9% efficiency under AM1.G 1-sun and 44.4% under 300-suns concentration have been demonstrated with InGaP/GaAs/InGaAs 3-junction solar cell by Sharp . Spectrolab has achieved 38.8% efficiency under 1-sun with 5-junction solar cells . FhG-ISE has demonstrated 46.0% under 58-suns concentration with 4-junction solar cells . Most recently, 39.2% under AM1.5 1-sun and 47.1% under 144-suns have been realized with 6-junction cell by NREL .
Radiation resistance and space applications of III-V compound single-junction and multi-junction solar cells
Development radiation-resistant solar cells is necessary for space application because solar cells degrade due to defect generation under radiation environment in space. Recombination centers tend to affect the solar cell performance by reducing the minority carrier diffusion length L in solar cell active layer from a pre-irradiation value L0 to a post-irradiation value Lφ through Eq.
where suffixes 0 and φ show before and after irradiation, respectively, Iri is introduction rate of i-th recombination center by electron irradiation, σi the capture cross section of minority-carrier by i-th recombination center, vth the thermal velocity of minority-carrier, D the minority-carrier diffusion coefficient, KL the damage coefficient for minority-carrier diffusion length, and φ the electron fluence. The III-V compound solar cells have better radiation tolerance compared to crystalline Si cells because many III-V compound materials have direct Band gap and higher optical absorption coefficient compared to Si with in-direct bandgap. In addition, InP-related materials such as InP, InGaP, AlInGaP, InGaAsP are superior radiation-resistant compared to Si and GaAs and have unique properties that radiation-induced defects in InP-related materials are annihilated under minority-carrier injection such as light-illumination at room temperature or low temperature of less than 100 K [38, 39].
Figure 17 shows calculated depth x distribution of carrier collection efficiency in Si, GaAs and InP under 1-MeV electron irradiation, calculated by using our experimental values [40, 41, 42] and Eq. (13), and by assuming carrier collection efficiency as a function of exp.(−x/L). It is clear from Figure 17 that GaAs has better radiation-tolerance and InP has superior radiation tolerance compared to Si.
Figure 18 shows changes in efficiency of Si single-junction, GaAs single-junction and InGaP/GaAs/Ge 3-junction space solar cells as a function of 1-MeV electron fluence. The InGaP/GaAs/Ge 3-junction solar cells is now mainly used for space as shown below because they are radiation-resistant and are highly efficient compared to Si and GaAs space solar cells .
Because GaAs single-junction solar cells and III-V compound multi-junction solar cells have high-efficiency and radiation-resistance compared to Si solar cells, III-V compound solar cells are mainly used in space as shown in Figure 19 .
The multijunction solar cells will be widely used in space because of their high conversion efficiency and good radiation resistance. However, in order to apply super-high-efficiency cells widely on Earth, it will be necessary to improve their conversion efficiency and reduce their cost. Figure 20 summarizes efficiency potential of single-junction and multi-junction solar cells, calculated by using the similar procedure presented in Section 2, in comparison with experimentally realized efficiencies under 1sun illumination. Altough single-junction solar cells have potential efficiencies of less than 32%, 3-junction and 6-junction solar cells have potential efficiencies of 42% and 46%, respectively.
The concentrator PV (CPV) systems  with several times more annual power generation capability than conventional crystalline silicon flat-plate systems will open a new market for apartment or building rooftop and charging stations for battery powered electric vehicle applications. Other interesting applications are in agriculture and large-scale PV power plants.
The multi-junction solar cells are greatly expected as high-efficiency solar cells into solar cell powered electric vehicles. Figure 21 shows required conversion efficiency of solar modules as a function of its surface area and electric mileage to attain 30 km/day driving. A preferable part of the installation is the vehicle roof. Because of space limitation for passenger cars, development high-efficiency solar cell modules with efficiencies of more than 30% is very important as shown in Figure 21 [46, 47]. In addition to high-efficiency, cost reduction of solar cell modules is necessary. Therefore, further development of high-efficiency and low-cost modules is necessary.
This chapter reviewed progress in GaAs-based single junction solar cells and III-V compound multi-junction solar cells and key issues for realizing high-efficiency solar cells. The III-V compound solar cells have contributed as space and concentrator solar cells and are expected as creation of new markets such as large-scale electric power systems and solar cell powered electric vehicles. Regarding single-junction solar cells, especially, GaAs solar cells have shown 29.1% under 1-sun illumination, highest ever reported for single-junction solar cells. In addition, analytical results for non-radiative recombination and resistance losses in III-V compound solar cells are shown by considering fundamentals for major losses in III-V compound materials and solar cells. Because the limiting efficiency of single-junction solar cells is 30-32%, multi-junction junction solar cells have been developed and InGaP/GaAs based 3-junction solar cells are widely used in space. The InGaP/GaAs/InGaAs 3-junction solar cells have recorded 37.4% under 1-sun and 44.4% under concentration. Recently, highest efficiencies of 39.1% under 1-sun and 47.2% under 144-suns concentration have been demonstrated with 6-junction solar cells. The 3-junction and 6-junction solar cells potential efficiencies of 42% and 46% under 1-sun, respectively. Further development of high-efficiency and low cost solar cells and modules is necessary in order to create new markets.
Our studies were partially supported by the NEDO (New Energy and Industrial Technology Development Organization) and JSPS (Japan Society for Promotion of Science). The author wishes to express sincere thanks to Dr. T. Takamoto, Sharp, Dr. K. Araki, Toyota Tech. Inst., Dr. M. Imaizumi, JAXA, Dr. A. Yamamoto, Fukui Univ., Dr. H. Sugiura and Dr. C. Amano, formerly NTT Lbs., Dr. SJ. Taylor, ESA, Prof. A. Kahn, South Arabama Univ., Prof. HS. Lee, Korea Univ., Prof. N. Ekins-Daukes, UNSW, Prof. A. Luque, UPM, Dr. A. Bett Dr. G. Sifer and Dr. F. Dimroth, FhG-ISE, Dr. M. Al-Jassim, Dr. R. Ahrenkiel and Dr. J.F. Geisz, NREL for their fruitful collaboration and discussion.
Conflict of interest
The author declares no conflict of interest.
- 1. Introduction
- 2. Analysis of non-radiative recombination and resistance losses of single-junction solar cells
- 3. Historical progress and key issues for high-efficiency III-V compound single-junction solar cells
- 4. Historical progress and key issues for high-efficiency III-V compound multi-junction solar cells
- 5. Radiation resistance and space applications of III-V compound single-junction and multi-junction solar cells
- 6. Future prospects
- 7. Conclusion
- Conflict of interest
A Brief Review of High Efficiency III-V Solar Cells for Space Application
J. Li, A. Aierken, Y. Liu, Y. Zhuang, X. Yang, J. H. Mo, R. K. Fan, Q. Y. Chen, S. Y. Zhang, Y. M. Huang and Q. Zhang
The demands for space solar cells are continuously increasing with the Rapid development of space technologies and complex space missions. The space solar cells are facing more critical challenges than before: higher conversion efficiency and better radiation resistance. Being the main power supply in spacecrafts, III-V multijunction solar cells are the main FOCUS for space application nowadays due to their high efficiency and super radiation resistance. In multijunction solar cell structure, the key to obtaining high crystal quality and increase cell efficiency is satisfying the lattice matching and bandgap matching conditions. New materials and new structures of high efficiency multijunction solar cell structures are continuously coming out with low-cost, lightweight, flexible, and high power-to-mass ratio features in recent years. In addition to the efficiency and other properties, radiation resistance is another sole criterion for space solar cells, therefore the radiation effects of solar cells and the radiation damage mechanism have both been widely studied fields for space solar cells over the last few decades. This review briefly summarized the research progress of III-V multijunction solar cells in recent years. Different types of cell structures, research results and radiation effects of these solar cell structures under different irradiation conditions are presented. Two main solar cell radiation damage evaluation models—the equivalent fluence method and displacement damage dose method—are introduced.
Space solar cells, being the most important energy supply unit, have been employed in spacecrafts and satellites for over sixty years since the first satellite was launched in 1958 . It has been developed from the initial single junction low efficiency silicon solar cells  to the now high efficiency multi-junction III-V compound multi-junction solar cells . The main objectives of space solar cell development are directed toward to improving the conversion efficiency and reducing the mass power ratio and increase the radiation hardness [4–7]. At present, the highest conversion efficiency of solar cells is 47.1% achieved by six-junction inverted metamorphic (6 J IMM) solar cells under 143 suns . The high-efficiency III-V triple-junction cells are also becoming the mainstream of space solar cells. The best research-grade multi-junction space solar cell efficiency so far is 35.8% for five-junction direct bonded solar cell and 33.7% for the monolithically grown 6 J IMM multi-junction solar cell [9, 10]. Despite the high fabrication cost, they offer excellent performance and reliable stability for space missions [11–13]. GaInP/GaAs/Ge (1.82/1.42/0.67 eV) lattice-matched triple-junction cells are well established with efficiencies of over 30% and fulfilled many space applications in the past two decades. However, the current mismatch between its subcells makes it difficult to improve the conversion efficiency further . New structures of current matched or lattice mismatched solar cell structures and different fabrication methods are proposed to overcome this problem, such as the metamorphic (MM) growth method , mechanical stack , wafer bonding technology , etc.
While improving the efficiency of space solar cells, the radiation resistance should also be considered. In-orbit solar cells suffer from irradiation damages due to high energy protons and electrons in the earth’s radiation belt and cosmic rays [18, 19], and consequently, the photoelectric performance of solar cells will be degraded. The main reason of the degradation of solar cell performance is due to the radiation-induced displacement damage in the solar cell lattice, resulting in a decrease in the lifetime of the photo-generated carriers [20–22]. Therefore, the degradation mechanism and performance of solar cells under an irradiation environment must be explored, and it is necessary to apply radiation hardening methods before the space mission starts. The degradation of electrical performance in solar cells directly affects the life-time of space missions. The researchers aimed at improving the radiation resistance of solar cells by adding a certain thickness of protective cover to the solar cell to shield the damage of certain particles , using back-surface (BSF)  or distributed Bragg reflector (DBR) , and thinning the base layer thickness of the current-limiting subcell , or using the p-i-n structure and different doping methods for multi-junction solar cells . The experimental observations show that annealing of the multi-junction solar cell can restore certain electrical properties after being radiated by high-energy particles .
In recent years, various new types of multi-junction solar cells with different combinations of materials have been developed by different research groups, and the expectations for future development are different. Solar cell conversion efficiencies are rapidly being updated, and scientists are still struggling to come up with solar cells which have high conversion efficiency and possess good radiation resistance. Although there are several reviews available which cover the manufacturing, efficiency, and application prospects of photovoltaic modules [29, 30], the new types of high efficiency space solar cells based on III-V compound materials have not been summarized yet. This review attempts to give a brief review on different types of space solar cells and emphasize the high energy particle irradiation effects of solar cells and recent results on the most promising types of solar cells, including dilute nitride, metamorphic, mechanical stack, and wafer bonding multi-junction solar cells.
Different Types of High-Efficiency Solar Cells
With the improvement of the manufacturing process and deposition technology of materials, the solar cells industry has developed tremendously. Solar cell materials are developed from a single material (single crystal Si, single-junction GaAs, CdTe, CuInGaSe, and amorphous Si:H) to compound materials, such as III-V multi-junction solar cells, perovskite cells, dye-sensitized cells, organic cells, inorganic cells, and quantum dot cells [31–33]. The structure of solar cells also forms homogeneous junction cell to heterogeneous junction solar cell, Schottky junction solar cell, compound junction solar cell, and liquid junction solar cell. In the purpose of its usage, it has also been developed from flat cells to concentrator cells and flexible cells [34, 35].
The silicon solar cells were used as the first choice in the spacecraft since the first solar-powered satellite was launched in 1958. The Soviet Space station, MIR, was launched in 1986, was equipped with 10 kW GaAs solar cells, and the power per unit area reached 180 W/m 2 . Then, the fabrication technique of GaAs-based cells experienced changes from Liquid Phase Epitaxy (LPE) to metal organic vapor phase epitaxy (MOVPE), from homogeneous epitaxy to heterogeneous epitaxy, from single junction to multi-junction structure [37–39]. Notably, their efficiency was continuously improved from the initial 16–25%, and over 100 kW industrial-scale power output per year has been reached . Higher efficiency reduces the size and weight of the array, increases the payload of the spacecraft and results in lower costs for the entire satellite power system. Therefore, GaAs-based solar cells are widely used in space systems and continue to be used today [41–43]. Comparing with silicon solar cells, GaAs solar cells have the following advantages :
(1) Higher photoelectric conversion efficiency.
(2) Direct-gap semiconductor materials.
(3) The Band-gap tailoring by controlling the composition and doping of material.
(4) Superior radiation resistance.
However, the processes involved in GaAs solar cell fabrication are complicated, and its cost is much higher than that of silicon solar cells owing to the expensive equipment and material preparation. Therefore, GaAs solar cells cannot be widely utilized in the civil market. Nevertheless, GaAs solar cells have gradually replaced silicon solar cells in the aerospace field, where higher cell efficiency and better radiation resistance are needed.
The loss in the efficiency of solar cells can be divided into two parts: the unabsorbed loss and excessive energy loss. When the photon interacts with the semiconductor materials, where the photon energy is smaller than the bandgap width, the valence Band electrons are not excited, and they do not generate an electron-hole pair to form an electrical current. However, when the photon energy is greater than the gap width, the excess energy is lost in the form of phonons or heat . Fortunately, multi-junction solar cells successfully solved this problem. Semiconductor materials with different Band-gaps are composed from top to bottom from large to small Band-gaps, and the higher energy photons are absorbed by the top large Band-gap material. The lower energy photons go through the upper large Band-gap material and reach the appropriate Band-gap width to generate power. Therefore, for multi-junction solar cells, finding a current matching and lattice matching cell material is the critical and general FOCUS [14, 45]. The following sections present a brief introduction of different types of multijunction solar cells in terms of their performance.
Lattice Matched GaInNAs Multi-Junction Solar Cell
In 1996, Kondow et al. demonstrated the epitaxial growth of 1.0 eV Band gap GaInNAs material with lattice matching to GaAs substrate, and applied it to fabricating infrared laser . Since then diluted nitride GaInNAs materials have been widely used in heterojunction bipolar transistors (HBTs)  and lasers , where GaInNAs HBTs base layer can reduce the open voltage and run under low working voltage. These features of diluted nitride GaInNAs materials are also useful in wireless communications and power amplifier applications. GaInAsN is a direct Band-gap semiconductor material, which can change its Band-gap by adjusting the component content of nitrogen and indium while keeping its lattice constant matching to conventional substrate materials such as GaAs and Ge. These advantages bring great potential for using 1.0 eV subcell in a high efficiency multijunction solar cell .
The Ga1-xInxNyAs1-y is used as a sub-cell material for GaInP/GaAs/GaInNAs/Ge four-junction solar cell by NREL . When y = 0.3x, the lattice constant of Ga1-xInxNyAs1-y matches GaAs and Ge, which is an ideal material to construct a GaInP/GaAs/GaInNAs/Ge (1.88/1.42/1.05/0.67 eV) four-junction solar cell with Band-gap matching. Figure 1A shows the representative GaInNAs devices structure grown by MOVPE. The device is grown with dimethylhydrazine (DMHy) as the nitrogen source. At the same time, the experimental results showed that the remaining factor of GalnAsN cell efficiency is 0.93 and 0.89 after 5 × 10 14 and 1 × 10 15 e/cm 2 electron fluence of 1 MeV electrons irradiation, respectively . The specific degradation of the device parameters is summarized in Table 1. The results showed that this type of cell structure possesses superior radiation resistance comparing to the traditional lattice matched multi-junction solar cell.
FIGURE 1. (A) The structure of GaInNAs device grown by MOVPE with dimethylhydrazine as the nitrogen source ; (B) The structure of InGaP/AlGaAs//Si triple-junction solar cell .
Thin-Film Solar Panels: Technologies, Pros Cons and Uses
The photovoltaic (PV) industry is led by traditional rigid crystalline silicon (c-Si) technology, featuring high efficiency, low price and higher availability, but this is not the only available option. Thin-film solar technology includes many features that make it unique for particular applications that are not suited for traditional c-Si PV modules.
There are many popular thin-film solar technologies available in the market, including Gallium Arsenide (GaAs), Cadmium Telluride (CdTe), and others, with new ones being researched and developed.
In this article, you will learn about the most important thin-film solar technology, its applications, advantages and disadvantages, and other interesting facts about the technology.
What are thin-film solar panels and why are they so important to the PV industry?
Thin-film solar panel technology consists of the deposition of extremely thin layers (nanometers up to micrometers) of semiconductors on backing materials that provide the body for a PV module. These materials generate electricity from solar radiation under the photovoltaic effect.
Traditional c-Si PV modules eclipsed thin-film solar technology in the past with higher efficiency for a decent cost, but this has been pairing up in recent years. Currently, c-Si technology features a better efficiency than most thin-film solar modules for a good cost, but thin-film solar technology is particularly suited for unique applications in the PV industry that make it irreplaceable by crystalline silicon.
A clear example is Gallium Arsenide (GaAs) technology. While it features an expensive cost, its high efficiency of up to 30% in Standard Testing Conditions (STC) and 68.9% in unique lab conditions, makes it ideal for concentrated PV (CPV) and space applications. Thin-film solar technology can also be used for flexible PV modules suited for various applications, Building Integrated Photovoltaics (BIPV), portable applications, and more.
The most Popular thin-film solar panel technologies and their applications
Thin-film solar technology is a compendium of different technologies including cutting-edge technologies, popular technologies used in commercial applications, and promising technologies being developed. In this section, we explain the most important thin-film solar technologies and their applications.
Gallium Arsenide (GaAs) Germanium (Ge): The most Popular thin-film for concentrated PV (CPV) and space applications
Gallium Arsenide (GaAs) and Germanium (Ge) are two of the most important thin-film solar technologies included in the category of multijunction III-V photovoltaics. These are complexly developed modules manufactured with several junctions instead of a single junction, designed to surpass the 33.5% Shockley-Queisser efficiency limitation set for single-bandgap solar cells.
GaAs and Ge thin-film solar cells are manufactured using Gallium and Arsenide for GaAs, and Germanium for the Ge PV modules. The III-V multijunction design in combination with the materials, increases the bandgap, resulting in higher electron mobility and saturated electron velocity, allowing these thin-film PV modules to absorb more energy from photons and deliver higher efficiency.
The major setback of GaAs and Ge thin-film solar cells is their high manufacturing cost and difficulty in growing for mass production. Even though this is a limitation, its high efficiency reaching up to 68.9% makes it uniquely suitable for space applications and concentrated photovoltaics (CPV).
Cadmium Telluride (CdTe) Copper indium-(Gallium)-Selenide (CIGS and CIS): The most popular thin-film for commercial applications
Cadmium Telluride (CdTe), Copper Indium-Gallium Selenide (CIGS), and Copper Indium Selenide (CIS) comprise another important group of thin-film solar technologies. The record efficiency is set at 22.1% for CdTe, 22.2% for CIGS, and 23.5% for CIS. They also feature a highly competitive cost per watt (/W).
Just like with other thin-film solar technologies, CdTe, CIGS, and CIS PV modules are manufactured by depositing thin layers of semiconductor materials using techniques like sputtering, evaporation, electrochemical deposition, and others. The backing material determines the flexibility of the module and therefore its application.
CdTe, CIGS, and CIS thin-film solar panels are not as popular as crystalline silicon for residential applications because of a lower efficiency and a larger space per watt required, but they are less expensive. The lower cost per watt makes these technologies uniquely qualified for solar power plants where installation space is not a limitation, but costs have to be brought down to a minimum.
The usage in commercial applications is the most important role that these thin-film solar technologies play in the PV industry. Technologies like CdTe, CIGS, and CIS are used to create electronic devices with embedded solar power generation, portable PV modules, BIPV, solar shingles, flexible PV modules for multiple applications, and more.
In the past, CdTe, CIGS, and CIS were not the only popular thin-film solar technologies used for commercial applications. Other important technologies that held a significant market share were Amorphous Silicon (a-Si) and Micromorph Silicon (μ-Si), but a failure to increase efficiency and reduce cost caused them to gradually disappear from the market.
Organic Thin-Film PV (OPV) Perovskites: Other important thin-film technologies being developed
Thin-film solar panels have not reached their peak, since the scientific community is still working on researching and developing new and more advanced technologies. The current trend under research includes organic thin-film PV (OPV) and tandem cells with a perovskite base, both holding a promising future in the PV industry.
OPVs are made using two semiconductor materials sandwiched together, with one of the layers being a conductive dye or organic semiconductor. This technology shows a promising future by delivering a low production cost and high stability and it could cause a Rapid shift in the PV market of the future if current limitations are overcome.
Tandem solar cells consist of a thin-film solar technology that stacks perovskite p-n junction layers on a base of crystalline silicon or other thin-film solar cells, showing a promising future for competing against traditional crystalline silicon due to its potential low cost and high efficiency. The record efficiency for tandem solar cells is currently set at 28.3% for c-Si-based cells, and at 26.2% for CIGS-based cells.
Challenges for tandem perovskite solar cells include water sensitivity, wide Band gap, uncontrolled crystallization, and others. OPV cells also have to be developed as large-size solar cells and solve a few other setbacks to hit the market. In the future, these thin-film solar technologies could replace rigid and other thin-film PV modules, by providing higher flexibility, lower costs, and lower weight for PV modules.
Pros cons of thin-film solar technology
Learning about the pros and cons for the different groups of thin-film solar technology is a great way to understand its unique features. In this section, we address each one of them.
Thin-film solar technologies like GaAs and Ge are able to deliver an astonishing performance, but for a higher cost. Other thin-film solar technologies like CdTe, CIGS, and CIS may require a large space to fit the same PV system that you would install with c-Si PV modules, but a better cost-efficiency and unique properties, make these technologies uniquely qualified for commercial applications.
As these technologies are further developed, future breakthroughs could increase their efficiency and reduce costs, making them more popular and increasing their market share. The following table illustrates the most important pros and cons for each group of thin-film solar technologies:
GaAs and Ge are among the best and most efficient thin-film solar technologies. These thin-film solar panels provide great efficiency and perform great in low and high-temperature climates, being uniquely suited for CPV and space applications. The major cons of these technologies are a high manufacturing cost and higher than normal solar cell degradation.
CdTe, CIGS, and CIS thin-film solar technologies have proved their worth in the PV industry. While less efficient than crystalline silicon, they have a better cost-efficiency ratio and are better for solar power plants. Their unique properties and low cost also make them ideal options for commercial applications like portable PV modules, BIPV, flexible solar panels, and others.
Tandem solar cells based in perovskite and OPV also have many advantages and great potential to impact the PV industry. The only inconvenience is that researchers have to find solutions to a few setbacks before these technologies can fully hit the market and be used for all types of commercial applications.
Thin-film solar panel market
The PV industry is mostly ruled by monocrystalline and polycrystalline silicon technology with a production share of around 95%. Thin-film solar technology is also a player in the PV industry, featuring a production share of 5% for usage in solar power plants, BIPV, space applications, regular rooftop PV installations, and more.
In 2021, the thin-film solar market was valued at 12.2 billion, and 14.7 billion dollars by 2022, or about 5% of the whole PV market. Additionally, in 3 years from 2018 to 2021, the gross world production (GWp) for CdTe thin-film solar grew threefold, becoming the most popular thin-film solar technology produced worldwide.
As thin-film solar technology keeps growing it is expected that the market share for this technology will develop even further in the PV industry. A study by Custom Market Insights estimates that by 2023 the thin-film solar industry could grow 74.82% up to 25.7 billion, holding almost 10% of the market share.
The most important applications of thin-film solar technology
Thin-film solar panels include several technologies with different characteristics and properties. In this section, we explain important applications for thin-film solar technologies like GaAs, Ge, CdTe, CIGS and CIS.
Building Integrated Photovoltaics (BIPV)
Building Integrated Photovoltaics (BIPV) can be used for façade, rooftops and glazing. This application replaces the rooftop, Windows (glazing), and façade of any building with aesthetically superior thin-film solar PV modules that fully integrate into the design of the building, providing it with the capacity to generate solar power for on-site use or to be exported to the grid.
The return on investment (ROI) for BIPV can be roughly estimated from 10 to 15 years, depending on the specifics of the system and location. BIPV systems can last for up to 30 years, supplying most or all of the power required to run a building. Popular technologies used for BIPV include CdTe, CIGS and CIS.
Concentrated photovoltaic (CPV) applications
Low to high-concentrated Photovoltaics or CPV uses optical devices to concentrate sunlight into the surface of PV modules. CPV can be used with any solar panel, but high-efficiency thin-film solar panels like GaAs and Ge are better for these applications since a PV module can produce 30% to 40% more energy than in regular conditions.
Spacecraft like satellites, space stations and rockets are exposed to radiation and limited weight can be carried out into space, making highly efficient and lightweight thin-film PV modules like GaAs and Ge, uniquely suited for these applications. While they are expensive technologies, it is more cost-efficient than carrying heavier-weight modules into space.
Thin-film solar technology like CdTe, CIGS and CIS features robustness, flexibility, low cost, and high efficiency making them better for portable applications. Some of these include foldable thin-film solar panels, solar phone chargers, solar flashlights, devices in general with embedded solar cells, and more. Future portable applications might include solar smartphones.
Public devices/equipment applications
Government and local authorities also take advantage of thin-film solar technology to install devices and equipment for public applications, making them independent from the grid and reducing their power consumption cost. Some of these applications include public Wi-Fi routers with solar panels, traffic lights operating with thin-film solar modules, solar street lights, and more.
Boats, RVs, buses and other vehicles also take advantage of solar energy thanks to thin-film solar technology. Some drivers carry portable thin-film solar panels in their vehicles, while others take it even further by installing flexible modules over the bow of boats, hoods or roofs of RVs, and more.
Rooftop PV installations
Thin-film PV installations are not as popular as c-Si ones, but they still happen. Some applications include thin-film technology based solar shingle installations and PV installation over business buildings, but mostly thin-film solar farms in utility-scale and industrial installations, where lower cost is important and space is not a limitation.
Final Word: Future and limitations of thin-film PV technology
Understanding the limitations and expected future of thin-film solar technology can be helpful in determining how this branch of the PV industry will develop. For instance, a-Si thin-film solar technology did not overcome efficiency and cost setbacks, making it shift out from the PV market in previous years. There are also concerns about toxic materials and scarcity of materials regarding thin-film solar products.
Surprisingly enough, there is also interesting news for thin-film solar technologies.
An important one is that certain thin-film solar technologies like GaAs may have future applications that go beyond the ground of solar power generation and step on the terrain of power transfer with the use of optics. Additionally, thin-film solar technologies using new materials might be developed in the future.
It has been estimated that the thin-film solar technology industry will grow by around 10% by 2030. With breakthroughs, the future may shine even brighter on thin-film solar technology, as it is further developed and takes on a higher market share in the PV industry.
Thin-Film Solar Technology (Guide)
In an industry that is constantly evolving, thin-film solar panels are an exciting and innovative product that can be used to efficiently convert sunlight into electricity.
Unlike the traditional, rigid monocrystalline or polycrystalline photovoltaic (PV) solar panels you may be used to seeing, thin-film solar cells are, well, thin and flexible.
Suitable for many unique applications, thin-film panels can be used to generate electricity in a variety of instances in which a traditional type of solar panel may be less effective.
To help you understand the pros, cons, strengths, and weaknesses of thin-film solar panels, let’s explore how they work and dive into some of the most exciting aspects of this emerging technology.
Definition of Thin-Film Solar
Thin-film solar panels harness energy from direct sunlight using one or more thin layers, or a thin film of semiconducting materials placed on a suitable base such as glass, plastic, or metal.
For an example that you are probably familiar with, solar-powered calculators are one of the most widely established applications for thin-film cells.
Thin-film solar cells can be made of a variety of materials, including popular compounds such as:
- Cadmium Telluride (CdTe)
- Copper Indium Gallium Diselenide (CuInSe2)
- Amorphous Silicon (a-Si)
- Gallium Arsenide
While thin-film solar products have been around for decades, the technology is advancing rapidly, with new ideas constantly being tested and improved. In early 2022, researchers at the University of Surrey successfully increased the energy absorption levels in a thin-film solar panel by 25%, achieving a new record of 21% efficiency.
Differences Between Thin-Film Solar Panels and Standard Silicon Solar Panels
The key differences between thin-film solar panels and standard silicon solar panels are their size, strength, and cost. Unlike bulky, rigid silicon solar panels, thin-film panels are as slim as a piece of paper, cheaper to produce, ship, and install, and can be flexible enough to mount on curved surfaces.
Today, traditional monocrystalline and polycrystalline photovoltaic (PV) solar panels are typically more efficient and durable than their thin-film counterparts. With less efficiency, a larger surface area may be required for thin-film cells to convert the same amount of sunlight into electricity as with standard silicon solar panels.
Still, as a lighter and cheaper option to produce and transport, continuous advancements in thin-film solar cells have allowed the technology to witness widespread adoption and a bright future ahead.
The Primary Thin-Film Solar Cell Materials
Ready to get technical? Here is a detailed look at the four main materials used in thin-film solar panels today:
Amorphous Silicon (a-Si) Solar Panels
As the first commercially available thin-film solar cell, Amorphous Silicon (a-Si) strips have been used since the late 1970s. Unlike the crystalline silicon wafers used in rigid panels, Amorphous Silicon cells generally have low efficiency levels but still perform well in a variety of light intensities.
Amorphous Silicon solar panels are made by depositing a layer of amorphous silicon onto a glass surface using chemical vapor deposition (CVD). The resulting material has a low thermal conductivity, which means it can absorb more heat than traditional crystalline silicon photovoltaic cells without overheating.
While cheap to manufacture and produce, a-Si panels tend to degrade more quickly than other types of thin-film solar panels, and have difficulty operating at temperatures below freezing.
Copper Indium Gallium Selenide (CIGS) Solar Panels
As one of the most popular thin-film technologies, CIGS solar cells use a series of copper, indium, gallium, and selenide layers to capture sunlight and generate electricity. CIGS panels utilize a multi-step process to collect and separate electrical charges, resulting in high-efficiency power production.
Suitable for building integration and several different flexible applications, CIGS research has created modules with thin-film solar panel efficiency levels up to 23% and rising, comparable to traditional solar panels. However, integrating copper, gallium, indium, and diselenide into one simple manufacturing process has made commercial production of the technology more difficult and expensive than other thin-film cells.
Cadmium Telluride (CdTe) Solar Panels
Second only to CIGS in popularity, cadmium telluride (CdTe) solar panels are another thin-film technology that has gained momentum in the last decade. Known for its quick and inexpensive development process, cadmium telluride solar panels have achieved similar efficiencies as traditional silicon solar panels, with reduced costs of production.
Flexible and ultra-thin, CdTe panels are among the most researched and tested technologies in new solar generation. However, the toxicity of the materials in CdTe solar panels has raised some environmental concerns.
Gallium Arsenide (GaAs) Solar Panels
With up to 40% efficiency in testing environments, Gallium Arsenide (GaAs) solar cells are another longstanding technology that is used in thin-film panels. Utilizing strong electric and heat resistant properties, GaAs solar panels have higher electron mobility than conventional silicon modules.
Tested and used in solar cars both on earth and in space (like the Mars Rover), GaAs solar cells are most applicable for high-power instances. While more expensive to produce than other thin-film technologies, GaAs solar cells continue to innovate and push the boundaries of renewable energy potential.
Advantages and Disadvantages of Thin-Film Solar Panels
Compared to traditional silicon solar collectors, thin-film solar panels come with a few distinct advantages and disadvantages.
Advantages of Thin-Film Solar Panels
- Lower Cost: Thin-film solar panels are generally cheaper to manufacture than traditional modules.
- Lighter Weight: Without any bulky or rigid parts, thin-film solar panels are easier to transport and install on a variety of surfaces.
- Flexible: With flexible arrays, thin-film solar panels can be installed on curved buildings, boats, walls, and more.
- Less Invasive: Unlike bulky silicon panels, some people consider thin-film panels less invasive and more visually appealing than large photovoltaic arrays.
Disadvantages of Thin-Film Solar Panels
- Less Efficiency: Generally less efficient than traditional panels, thin-film installations require more space to produce the same amount of electricity.
- Reduced Durability: Built for flexibility, thin-film solar panels may be more prone to cracks, breaks, and malfunctions from weather conditions like rain or snow.
- Newer Technology: The testing, manufacturing, and real-world applications of thin-film solar cells are still very limited compared to rigid PV panels.
Best Thin-Film Solar Manufacturers
As one of the fastest-growing sectors of the renewable energy industry, there are many leading manufacturers currently pursuing thin-film solar products. While formerly leading companies like Solar Frontier have moved away from the space, there are still many thin-film solar companies to watch in the coming years:
- Hanergy:Hanergy is one of the largest solar manufacturers in the world, and specializes in thin-film solar panels. With six RD centers in Beijing, Sichuan, Silicon Valley, and Uppsala, Sweden, Hanergy has made significant investments in thin-film solar cell research, resulting in almost 1,000 patents in new energy, including copper indium gallium selenide (CIGS) technology that has reached 21% efficiency.
- Renogy: With a wide range of flexible solar products, Renongy is a consumer-facing company for small-scale electricity production. Today, their thin-film solar panels can be purchased one by one, or at wholesale rates for large installations.
- SunPower: As one of the largest solar panel manufacturers in the world, SunPower’s flexible solar panels are portable, flexible, and backed by a thick, weather-resistant copper foundation. The California-based company currently sells thin-film solar panels primarily for use on the go in RVs and other small applications.
Exciting Developments in Thin-Film Solar Panels
With a strong foundation powered by decades of research and development, thin-film solar cells are among the most exciting and innovative technologies driving the future of solar power. While we may still be simply scratching the surface of their full potential, here are a few interesting advancements to look out for in the near future:
- Researchers at Stanford Oxford University have developed a new type of solar cell that uses organic molecules called perovskite solar cells, which may be cheaper and easier to produce than traditional silicon-based cells.
- Similarly, researchers are developing a working perovskite “solar paint” which can be sprayed, printed, or dyed onto a surface to conduct electricity.
- The future is also bright for thin-film building-integrated photovoltaics, such as transparent solar panels and solar shingles. In both residential and commercial applications, these technologies can bring the electricity generation of thin-film solar into the functional elements of a building.
- Looking out even further, the success of thin-film solar panels in space makes the lightweight and highly efficient technology a key element in further Galaxy exploration.
Should you get thin-film solar panels for your home’s roof?
Thin-film solar panels are currently most often utilized on commercial buildings where ample space is available since many residential roofs are limited in total surface area. With that said, technological advancements are continuing to push the efficiency of thin-film panels forward, and residential applications are slowly becoming more cost-effective.
There are many pros and cons of buying flexible solar panels and the choice to use thin-film cells should be weighed on a case-by-case basis.
The Future of Thin-Film Solar Panels
With versatility and ease of use, thin-film solar panels are among the most exciting developments in the solar industry. As the technology continues to advance, thin-film solar cells are being used in many practical applications, beyond just rooftop power generation.
If you’re considering a solar panel installation of any kind, you can talk to Palmetto to learn more about your options. With our Free Solar Design and Savings Estimate tool, you can instantly see how much you can save with solar energy.
Types of Solar Panels
All solar panels are not the same. They differ in performance, appearance, price, material, application, and size. The types of solar panels you need for your home or office depends on the roof size, consumption, budget, efficiency, among other factors.
There are 3 common kinds of solar panels:
- Monocrystalline solar panels
- Polycrystalline solar panels
- Thin-film solar panels
Although there are other types of solar panels, most are not economically or technologically viable.
The types of solar panels are classified into 3 groups. The classification is based on the kind of materials used and the commercialization of the product.
- First-generation solar panels
- Second-generation solar panels
- Third-generation solar panels
First Generation Solar Panels
Monocrystalline and polycrystalline solar panels fall under this category. The cells are made of crystalline silicon and gallium arsenide (GaAs) wafers. They are the most common types of solar panels in commercial and residential solar panel installation. Because of their widespread use, they are also referred to as conventional or traditional solar panels.
First-generation solar panels are the oldest PV cells, and their fabrication and technological applications are well-known. GaAs is a better material than silicon because it has higher optical properties. Therefore, it requires thicker silicon wafers to harness the same amount of energy as GaAs.
But, gallium and arsenide are expensive and not commercially viable for the manufacture of solar panels. The materials are limited on the surface of the earth. Therefore, silicon remains the primary material in the manufacture of solar panels.
Let’s have a look at each of the solar panel types under the first generation.
Monocrystalline Solar Panels
The solar cells are made of the purest form of silicon. They have a uniform silicon composition, which gives them high efficiency. They have rounded edges because silicon crystals are cylindrical. You can identify the panels from the even rows and columns.
The silicon wafers used in monocrystalline cells have high efficiency (up to 20%) compared to other types of solar panels. Therefore, you require fewer monocrystalline solar panels; this makes them ideal for use in small-sized roofs. You can also use this type in pole mounts because the space is also limited.
However, the price of monocrystalline solar panels is higher. They are more costly to manufacture than the other types. The solar panels have a longer lifespan because of increased resistance to temperatures; thus, a more extended warranty. The monocrystalline solar panel system could last for more than 30 years.
Polycrystalline Solar Panels
Do you want to install cheap solar panels, and you have unlimited roof space? Polycrystalline solar cells have lower efficiency but are feasible for residential buildings where space may not be a problem. The panels are also referred to as multi-crystalline solar panels.
Although they are made from the same material as monocrystalline, they have lower efficiency, ranging between 15-17%. The solar panels have a speckled bluish color, which many homeowners consider unattractive. Another difference from the former type is the appearance. Polycrystalline solar panels have sharp wafer edges because of how they are manufactured.
A decade ago, polycrystalline solar panels were the most common type of solar panels. However, their popularity has dwindled because of low efficiency. The average capacity of an average polycrystalline solar panel system is approximately 300 watts. Therefore, you require around 20 for a 6 kW solar panel system.
The life expectancy of polycrystalline solar panels is lower. Thus, a shorter warranty period than monocrystalline solar panels. The choice between polycrystalline and monocrystalline solar panels is not outright. Each has its ideal application, depending on your situation. You should go for multi-crystalline solar panels if you want to cut on cost and the size of your roof or ground mounts is not limited. However, the panels are affected by high temperatures, which can lower their lifespan.
Second Generation Solar Panels
Thin-film solar panels make up the second generation of solar panels. some of the most common types of 2 nd generation solar cells include:
- Amorphous silicon solar panels
- Cadmium telluride (CdTe)
- Copper indium gallium selenide (CIGS)
- Concentrated photovoltaic cells (CVP)
Thin-film solar panels have lower efficiency than the crystalline types because of the material used. They are common in utility-scale applications where space is plenty.
Amorphous silicon (a-Si) solar panels are made of hydrogenated silicon, which has low energy conversion efficiency. The material is deposited in flexible substrates like metal, plastic, and glass. The solar panels are less durable compared to crystalline silicon cells; thus, a shorter warranty period.
CdTe solar panels are made from semi-conductors pressed between thin films of glass. There are concerns about cadmium safety, but studies show that a compound of the two elements has lower toxicity than Cd alone. Therefore, proper disposal of the material is advisable to prevent any adverse health effects. This type of solar panel is the most common in commercial thin-film applications.
Copper indium gallium selenide
CIGS solar panels are an exciting option because of their high efficiency. However, the cost of manufacturing solar cells makes them an expensive option. It is difficult for copper indium gallium selenide solar panels to compete with crystalline silicon cells. However, the solar panels have a higher efficiency than other kinds.
Thin-film solar panels are the most flexible. They can adopt different shapes for aesthetic value. There are many studies to improve solar panels’ efficiency and overcome the commercial and technological barriers of the solar cells.
Concentrated photovoltaic cells
CPV is a new technology that uses curved mirrors and lenses to concentrate sunlight to highly efficient solar cells. The solar panels can achieve an efficiency of up to 41%, which is double what the second most efficient type can harness. The technology’s commercial application will be a significant breakthrough in solar energy because it will reduce the cost and space required to install solar panels.
Third Generation Solar Panels
There is a limit to the efficiency of solar panels. Shockley-Queisser ranges between 31-41% for a single bandgap solar cell. The third-generation of solar panels seeks to overcome this limit and improve efficiency. The main objective of the technology is to convert solar cell non-compatible light frequencies to compatible frequencies.
There are promising products under development that could make solar energy more efficient. The solar panels seek to tap into the strengths of crystalline silicon and the 2nd generation PV technology. The most advanced third-generation solar panels include:
Which Type of Solar Panels Should I Buy?
There is no direct answer to this question without an evaluation of your situation. Some of the essential factors that affect the type of solar panels for your commercial or residential installation include:
- Size of the roof or ground mounting space.
- Aesthetic preferences.
- Size of the solar panel system.
Monocrystalline solar panels are the most ideal if you have limited space. On the other hand, polycrystalline cells are suitable when low on the budget. Thin-film solar cells are the most common in power purchase agreements because of the short lifespan. They are also ideal for utility-scale or communal solar energy installations.
Solar energy technology undergoes drastic changes in a short period because it is a developing technology. There are many feasibility studies to evaluate the application of different solar panels under review. Therefore, what is efficient today might be outdated within a year. You should keep an eye on the industry and do thorough research before settling for any solar panels.